1. Field
The present disclosure relates to internal combustion engines and, more particularly, to apparatus and methods for varying the valve lift of a valve in an internal combustion engine by varying the fulcrum point of a rocker arm actuating the valve in order to vary the amount of valve lift over a wide range.
2. Background
In internal combustion engines, spring closed poppet-type valves contained in the cylinder head of a typical engine are designed to provide sufficient spring closing force given a full power or maximum RPM operating condition. This ensures that the poppet-type valves, which are the intake and exhaust valves, have sufficient acceleration to close quickly enough when the engine revolutions are high (i.e., a full or near full load condition). Additionally, it is known to design engines to ensure that the valve lift displacement of the poppet-type valves is sufficient for full load conditions. Accordingly, rocker arms, which are part of the mechanism for converting the rotational motion of a camshaft to motion for actuating the poppet valves, are configured with a rocker arm ratio of typically from 1.5:1 to 1.7:1 for maximum valve lift, which effectively extends the cam lift profile by that ratio.
By designing and operating an engine for full power conditions, however, excessive energy is consumed during less than full power condition, such as during engine idle conditions, where a substantial amount of the fuel consumed is simply used to rotate the camshaft, which in turn drives the rocker arms and poppet-type valves. This is due to the fact that the magnitude of full valve spring compression is substantial given that the poppet-type valves are being fully opened, thus requiring maximum force be applied for each valve opening event.
In most applications of internal combustion engines, however, operating conditions typically necessitate low to moderate power output with an according reduced fuel flow or airflow to the engine. With reduced fuel flow or airflow, lower valve lift is adequate for engine operation and full valve lift is unnecessary. Accordingly, in order to reduce energy consumption in internal combustion engines it is known to vary or lessen the valve lift to the tailored demands of intake and exhaust flow in and out of the cylinders. That is, by reducing the parasitic drag caused by valve spring compression, the caloric and thermal efficiency or net torque output of the engine may be improved. In order to reduce this drag, various mechanisms are known to either lessen or limit valve lift during periods of light engine load or even shut down or deactivate a rocker arm. Such mechanisms, however, typically add complexity to an engine, as well as height to the head portion of an engine. For example, a known approach includes an eccentric rotatable shaft and accompanying lever located above a rocker arm to vary a fulcrum point of the rocker arm. This arrangement, however, adds height to the engine block and complexity to the engine with the addition of moving parts.
Additionally, it is important when reducing valve lift to ensure that such variation of the valve lift does not result in lost motion and an increase in lash, which some known approaches do not adequately provide. Further, the mechanisms for varying valve lift should maintain push rod tightness, in the case of a push rod type engine, and should not change valve timing, which some known approaches fail to provide. Another consideration when reducing valve lift is to maintain the strike effect (i.e., what is known as “Long arm” strike effect) such that the tip of a rocker arm substantially maintains the same positioning with respect to the valve stem.